Benzene sensors using metal oxides and associated methods

ABSTRACT

In an embodiment, a method for fabrication of VOC sensor comprises dissolving one or more metal precursors in a reagent to form a solution, adding a reducing agent to precipitate a metal oxide compound, subjecting the solution to acoustic energy, recovering a nanoscale metal oxide, and forming a sensing layer in a chemo-resistance sensor using the nanoscale metal oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/081,657 (entitled GAS-PHASE BENZENE SENSOR AND ASSOCIATED METHODS filed Nov. 19, 2014), which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Detection of benzene and similar volatile organic compounds (VOC) is of high importance for safety and process control in chemical, petrochemical, steel and other manufacturing industries, as well as for minimizing environment pollution with these harmful gases. Benzene (C₆H₆) is a highly flammable, toxic, human carcinogen, organic hydrocarbon. It is widely used as an intermediate in processes leading to plastics, nylon, lubricants, coke, fertilizers, detergents, etc. In recent years, in many regions, including US and EU, benzene has replaced lead in gasoline composition. Due to its increased harmful potential, severe regulations have been imposed against its industrial use. In EU, gas can contain maximum 1% benzene by volume, while in US the upper limit is 0.62%. Monitoring benzene concentration is a vital requirement for the personal protection of people working in oil and gas storage and transportation, oil refineries, petrochemical industry. At concentration levels higher than 10,000 ppm, benzene can be lethal, while repeated exposures at much lower levels can lead to cancer, heart and brain failures, and endocrine diseases. The level over which benzene becomes harmful is currently set at the threshold limit value (TLV) of 0.5 ppm.

Currently, benzene sensing is performed by employing several techniques: multi-gas monitors, metal-oxides (MOx) based chemo-resistors, electrochemical detectors, fixed or portable gas chromatographs, single gas (colorimetric) detection tubes, and/or photoionization detectors (PIDs). A combination of the last two technologies leads to Ultra RAE3000, a portable benzene and compound-specific VOC monitor commercialized by Honeywell's RAE Systems. Ultra RAE3000 employs a PID, a low energy UV lamp and pre-filter tubes. Honeywell top-solution has an accuracy of +/−10%.

Other sensors that are commercially available for such industrial applications, as well as breath alcohol portable detectors, include a thick film of SnO₂ deposited on ceramic substrate, which is heated on the other side by a platinum heater. Even if this sensor is recommended not only for domestic applications, but also for portable applications, it is consuming about 660 mW for heating the substrate to the optimum sensing temperature and reading the detector response. Such a level of power consumption is determining a frequent battery replacement in portable applications, which may raise safety issues in the field operation.

In addition, the above noted sensors are detecting these VOC's gases only at relatively high concentrations, above 50 ppm, while the present requirements for benzene in the ambient are as follow: the threshold limit value (TLV) is 0.5 ppm, the short term exposure limit (STEL) is 2.5 ppm, while the immediately dangerous to health and life (IDHL) level is 500 ppm. Therefore, in safety applications, it is useful to detect much lower gas concentrations and then give an alarm and take an early stage action against any hazardous situation. Therefore, there is a strong motivation for increasing the sensitivity of the existing commercial sensors, as well as decreasing power consumption of VOC sensors so that an electric power much below 100 mW to be used and concentrations much below 50 ppm to be detected for VOC gases.

It is already largely accepted by the business and scientific community that the use of nanostructured sensing materials is increasing the sensitivity due to its material architecture and it is allowing the reduction of the power consumption, due to their large specific area and increased porosity, which are thus increasing the number of active sensing sites, while their surface energy is high enough for the sensing reactions to take place without too much thermal energy added from outside.

SUMMARY

In an embodiment, a method for fabrication of VOC sensor comprises dissolving one or more metal precursors in a reagent to form a solution, adding a reducing agent to precipitate a metal oxide compound, subjecting the solution to acoustic energy, recovering a nanoscale metal oxide, and forming a sensing layer in a chemo-resistance sensor using the nanoscale metal oxide.

In an embodiment, a VOC sensor comprises a substrate, a plurality of leads disposed on the substrate, and a metal oxide nanocomposite film disposed in electrical contact with at least two of the plurality of leads.

These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

FIG. 1 schematically illustrates another benzene sensor according to an embodiment.

DETAILED DESCRIPTION

It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims along with their full scope of equivalents.

The following brief definition of terms shall apply throughout the application:

The term “comprising” means including but not limited to, and should be interpreted in the manner it is typically used in the patent context;

The phrases “in one embodiment,” “according to one embodiment,” and the like generally mean that the particular feature, structure, or characteristic following the phrase may be included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention (importantly, such phrases do not necessarily refer to the same embodiment);

If the specification describes something as “exemplary” or an “example,” it should be understood that refers to a non-exclusive example;

The terms “about” or approximately” or the like, when used with a number, may mean that specific number, or alternatively, a range in proximity to the specific number, as understood by persons of skill in the art field; and

If the specification states a component or feature “may,” “can,” “could,” “should,” “would,” “preferably,” “possibly,” “typically,” “optionally,” “for example,” “often,” or “might” (or other such language) be included or have a characteristic, that particular component or feature is not required to be included or to have the characteristic. Such component or feature may be optionally included in some embodiments, or it may be excluded.

Disclosed herein are two VOC sensors that can detect benzene. In an embodiment, a nanocomposite metal oxide can be formed and used in a chemo-resistor sensor. In this sensor, a metal oxide film or layer can be disposed over two interdigitated electrodes. A resistance detected between the two electrodes can be changed by the presence of benzene reacting with the metal oxide. The resistance can increase or decrease based on the specific metal oxide used. A sonochemical synthesis method can be used to form the metal oxides used in the sensor described herein. This preparation technique can produce a nanoscale metal oxide material having a relatively high surface area. By increasing the surface area relative to other metal oxide detectors, the operating power can be reduced to provide a similar detection level. This may provide a sensor having a longer battery life.

FIG. 1 schematically illustrates a cross-sectional view of a chemo-resistor sensor 100. The sensor 100 generally comprises a substrate 102 having two or more electrically conductive leads 104, 106 and a sensing layer comprising a semiconductor film 108 in electrical contact with the leads 104, 106 (e.g., metal electrodes, etc.). The substrate 102 can be electrically insulating (e.g., silicon dioxide, alumina, polymer, etc.). The leads 104, 106 can be disposed adjacent one another, and in some embodiments, the leads can be interdigitated. A housing 112 can be disposed about the various elements of the sensor 100 to protect the internal components and provide a controlled entrance for the various chemical compounds to enter and react at the semiconductor film surface. For example, an aperture 114 may control the exposure of the semiconductor film 108 to a component such as benzene present in the atmosphere adjacent the sensor 100.

The leads 104, 106 and/or the heating element 110 can be coupled to a suitable control and detection circuitry. The control circuitry can be configured to detect a resistance between the leads 104, 106, which can be affected by one or more reactions occurring in the semiconductor film 108. Semiconductor manufacturing techniques such as sputtering, vapor deposition, masking, and the like can be used to deposit the leads 104, 106 and the optional heating element 110 on the substrate 102. Any suitable film deposition techniques such as maskless direct printing, screen printing, or the like can be used to dispose the semiconductor film 108 on the sensor 100, where one method is described in more detail below.

An optional heating element 110 can be disposed in thermal contact with the substrate 102. Since the substrate 102 can also be thermally insulating, the heating element 110 can be disposed on the side of the substrate with the leads 104, 106 and the semiconductor layer 108. For example, the heating element 110 can comprise a resistive heating element interdigitated with the leads 104, 106.

In an embodiment, the sensing metal oxide semiconductor film 108 can be formed from metal oxide nanocomposites. The sensing layer can comprise different metal oxide combinations to which noble metal can be added, and in some embodiments, a sonochemical synthesis method can be used for the preparation of the sensing layer.

The use of the sonochemical synthesis method can allow for a one-pot synthesis. Further, this preparation method has the advantage that the layer (nano)structuring can be controlled by the value of power and intensity of acoustic radiation to be applied during cavitation-activated chemical reactions between desired precursors and reagents (like CTAB or triblock copolymer P123), the last ones having a major role in guiding the nanostructuring (nanowires, nanoflowers, nanofibers, etc).

The result of the sonochemical synthesis is a metal oxide nanocomposite, which may be present as a powder of nanostructured metal oxide. The powder can be collected at the end of process, by washing, filtrating and drying the reaction products. The nanostructured powder can then mixed with a binder to provide a slurry of controlled viscosity. The slurry can then be deposited as a thick or thin sensing film on the leads 104, 106 (e.g., interdigitated metal electrodes deposited on the substrate 102). After thermal consolidation of the sensing layer to remove the organic binder, the chemo-resistor can then be used for gas detection, including the detection of various VOC gases including benzene.

In some embodiments, a noble metal can be added to the metal oxide nanocomposite as a doping. The noble metal doping can serve as a catalyst to activate the oxidation of short and long chain hydrocarbons, thereby allowing the reactions to take place at much lower temperatures than in the ambient. In some embodiments, the metal oxides can also act as catalysts (and a source of lattice oxygen in the case of ceria for preventing noble metal sintering) as well as semiconductor support for the charge transfer reactions.

In an embodiment, the sonochemical synthesis method can include the use of various metal precursors being dissolved in a reagent. The metal precursors can include nitrates of copper, chrome, cobalt, cerium, aluminum, manganese, or any combination thereof. If a noble metal is to be included, a precursor comprising a noble metal such as platinum, gold, palladium, rhodium, iridium, ruthenium, silver, or any combination thereof. For example, if platinum is being included in the metal oxide, hexachloroplatinic acid can be included with the precursors.

The reagents can include any solvent suitable for dissolving the precursors, and can include, but are not limited to, water, ethanol, a reducing agent (e.g., hydrazine, sodium hydroxide, urea, etc.), or any combination thereof. Surfactants can be added to order the resulting precipitates into the desired shapes. Examples of suitable surfactants include, but are not limited to, cetyltrimethylammonium bromide (CTAB) (C₁₉H₄₂BrN), oleyl amine (C₁₈H₃₇N), or any combination thereof.

The precursors, the reagents, and the surfactants can be combined and an initiator such as a basic solution of sodium hydroxide or urea can be added to the solution dropwise. The resulting solution and/or slurry can be subjected to acoustic waves to produce the desired precipitate particle size. In an embodiment, the solution can be subject to between about 50 W and 150 W of acoustic energy for between about 10 minutes and about 5 hours as part of the sonochemical treatment. The resulting precipitate can then be washed and dried in an oven. The powder can then be calcined at between about 350° C. and about 650° C. to produce the nanocomposite metal oxide powder having nanoscale dimensions (e.g., between about 1 nanometers and about 500 nanometers).

Metal oxide nanocomposite thin films useful as the sensing layer can be obtained by mixing the nanocomposite powders with water-glycerol-bicine solution for getting a nanoink with controlled rheological properties so that to be compatible with maskless direct printing tool like that provided by “OPTOMEC” or “Nanoink”. In order to remove the organic additives, film drying and firing in air at about 500° C. to about 600° C. can be used. The resulting film can serve as a semiconductor layer used to detect organics based on a reaction at the materials.

In some embodiments, screen printing or other techniques can be used to provide a thick or thin film on the substrate over the leads 104, 106. For example, a thick film fabrication method for chemo-resistive VOC gas detection consists in mixing the metal oxide nanocomposite powders described above with terpineol for making a paste which can be screen printed on the electrode structure and thermally treated at 500° C.-600° C. in air to produce a thick solid film for the sensor 100.

The resulting semiconductor material can comprise a relatively high surface area. The ability to increase the surface area relative to other metal oxide layers may allow for a greater rate of reaction, and thus resistance change, which can be detected at lower power levels. In addition, the temperature is usually increased to increase the reaction rate. By providing a larger surface area, a detectable resistance change can be produced at a lower temperature, thereby reducing the overall power requirements for the sensor.

EXAMPLES

The disclosure having been generally described, the following examples are given as particular embodiments of the disclosure and to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification or the claims in any manner.

Example 1

In this example, sonochemical synthesis is used to prepare CuO—Cr₂O₃ nanocomposite powders from metal nitrates. The components include: precursors: copper nitrate Cu(NO₃)₂*3H₂O, chromium (III) nitrate Cr(NO₃)₃*9H₂O; reagents: solvent: H₂O, ethanol, reducing agent: hydrazine (N₂H₄) or sodium hydroxide (NaOH) or urea CO(NH₂)₂; and surfactant: cetyltrimethylammonium bromide (CTAB) (C₁₉H₄₂BrN) or oleyl amine (C₁₈H₃₇N).

The preparation method is carried out as follows:

1. Dissolve appropriate amount copper nitrate in water;

2. Dissolve appropriate amount of chromium nitrate in water;

3. Add the solution from step 2 to the solution from step 1;

4. Dissolve the appropriate amount of CTAB or oleyl amine in water;

5. Add the CTAB or oleyl amine solution from step 4 to the mixture of dissolved copper nitrate and chromium nitrate;

6. Dissolve the hydrazine in water;

7. Alternative, prepare an aqueous solution of NaOH or urea;

8. Add drop wise the CTAB (or oleyl amine) solution to the solution of dissolved copper nitrate and chromium nitrate;

9. Alternative, add drop wise the NaOH (or urea) solution to the solution of dissolved copper and chromium nitrates;

10. Stir the final solution for 10 minutes and then expose it to the high power (100-200 W) high acoustic intensity sonochemical treatment for about 3 hours;

11. Separation and washing (in H2O and ethanol) of the resulting powder;

12. Dry the powder in an oven at a temperature of 100° C.-120° C.

13. Calcination of the dried powder in an oven, at a temperature of in the range from 350° C. to 650° C.;

14. Use the above calcinated powder for sensing layer.

Example 2

In this example, sonochemical synthesis is used to prepare a Co₃O₄—CeO₂ nanocomposite powder from cobalt and cerium nitrates for VOC detection.

The component include: precursors: Cobalt (II) nitrate Co(NO₃)₂.6H₂O and cerium (III) nitrate Ce(NO₃)₃.6H₂O; chemical reagents: ethanol, reducing agent: NaOH (or urea) and surfactant: CTAB (or oleylamine:C₁₈H₃₇N).

The preparation method is carried out as follows:

1. Dissolve the appropriate amount of cobalt nitrate in ethanol, while stirring for about 1 hour;

2. Dissolve the appropriate amount of cerium (III) nitrate in ethanol, while stirring for about 1 hour;

3. Mix the solutions from steps 1 and 2;

4. Dissolve the appropriate amount of CTAB (or oleyl amine) in water;

5. Add in a drop wise manner the CTAB (or oleyl amine) solution to the mixture from step 4, to dissolved nitrates mixture from step 3 while stirring;

6. Dissolve the appropriate amount of NaOH (or urea) in water;

7. Add in a drop wise manner the dissolved metal nitrates-CTAB (oleyl amine)-solution obtained at step 5 to the NaOH (or urea);

aqueous solution from step 6 while stirring;

8. Stirring of the final solution of metal nitrates-CTAB (or oleyl amine)-NaOH (or oleyl amine) for about 10 minutes;

9. Expose the mixture from step 8 to high power (100-200 W) high intensity acoustic radiation for about 3 hours;

10. Separation of the resulting powder and washing it in ethanol water mixture;

11. Drying the resulting power in an oven at temperature of about 100° C.-120° C.;

12. Calcination of the dried powder at a temperature of about 500° C.-600° C.;

13. Use of the dried calcinated powder for VOC sensing film preparation.

Example 3

In this example, sonochemical synthesis is used to prepare a CeO₂—MnO_(x) nanocomposite powder from cerium and manganese nitrates for VOC detection. The component include: precursors: cerium nitrate hexahydrate Ce(NO₃)₃.6H₂O and manganese (II) nitrate Mn(NO₃)₂; solvent: H₂O, ethanol, reducing agent: NaOH or urea; and surfactant: CTAB or oleyl amine.

The preparation method is carried out as follows:

Dissolve the appropriate amount of cerium III nitrate in ethanol, while stirring for about 1 hour;

Dissolve the appropriate amount of manganese nitrate in ethanol, while stirring for about 1 hour;

Mix the solutions from steps 1 and 2 so that to obtain Ce/Mn molar ratio=1/7 to 5/7;

Dissolve the appropriate amount of CTAB (or oleyl amine) in water;

Add in a drop wise manner the CTAB (or oleyl amine) solution to the mixture from step 4, to dissolved nitrates mixture from step 3 while stirring;

Dissolve the appropriate amount of NaOH (or urea) in water;

Add in a drop wise manner the dissolved metal nitrates-CTAB (or oleyl amine)-solution obtained at step 5 to the NaOH (or urea) aqueous solution from step 6 while stirring;

Stirring of the final solution of metal nitrates-CTAB (or oleyl amine)-NaOH (or urea) for about 10 minutes;

Expose the mixture from step 8 to high power (100-200 W) high intensity acoustic radiation for about 3 hours;

Separation of the resulting powder and washing it in ethanol water mixture;

Drying the resulting power in an oven at temperature of about 100° C.-120° C.;

Calcination of the dried powder at a temperature of about 500° C.-600° C.;

Use of the dried calcinated powder for VOC sensing film preparation as it will be described below;

The material was calcinated at 550° C. for 4 hours.

Example 4

In this example, sonochemical synthesis is used to prepare a CeO₂—MnO_(x) nanocomposite powder from cerium and manganese nitrates for VOC detection. The component include: Precursors: cerium nitrate, Ce(NO₃)₃.6H₂O, aluminum nitrate nonahydrateAl(NO₃)₃.9H₂O and hexachloroplatinic acid H₂PtCl₆*6 H₂O; and chemical reagents: solvent: distilled water (H₂O), ethanol, reducing agent: sodium hydroxide (NaOH) or urea, and surfactant: CTAB or oleyl amine.

The preparation method is carried out as follows:

Dissolve the appropriate amount of cerium III nitrate in water, while stirring for about 1 hour;

Dissolve the appropriate amount of aluminum nitrate in water, while stirring for about 1 hour;

Dissolve the appropriate amount of H₂PtCl₆*6 H₂O in water, while stirring;

Mix the solutions from steps 1-3 so that to obtain (0.5-1.5) wt % Pt in (Al₂O₃-30 wt % CeO₂);

Dissolve the appropriate amount of CTAB in water;

Add in a drop wise manner the CTAB or oleyl amine) solution to the mixture from step 5, to the dissolved nitrates-Pt mixture from step 4 while stirring;

Dissolve the appropriate amount of NaOH (or urea) in water;

Add in a drop wise manner the dissolved metal nitrates-CTAB (or oleyl amine)-solution obtained at step 5 to the NaOH (or urea) aqueous solution from step 7 while stirring;

Stirring of the final solution of metal nitrates-Pt-CTAB (or oleyl amine)-NaOH (or urea) for about 10 minutes;

Expose the mixture from step 9 to high power (100-200 W) high intensity acoustic radiation for about 3 hours;

Separation of the resulting powder and washing it in ethanol water mixture;

Drying the resulting power in an oven at temperature of about 100° C.-120° C.;

Calcination of the dried powder at a temperature of about 500° C.-600° C.;

Use of the dried calcinated powder for VOC sensing film preparation.

Having described the various systems and methods herein, various embodiments can include, but are not limited to:

In a first embodiment, a method for fabrication of VOC sensor comprises: dissolving one or more metal precursors in a reagent to form a solution, adding a reducing agent to precipitate a metal oxide compound, subjecting the solution to acoustic energy, recovering a nanoscale metal oxide, and forming a sensing layer in a chemo-resistance sensor using the nanoscale metal oxide.

A second embodiment can include the method of the first embodiment, further comprising: adding a surfactant to the solution prior to subjecting the solution to the acoustic energy.

A third embodiment can include the method of the second embodiment, wherein the one or more metal precursors comprise copper nitrate and chromium nitrate, wherein the nanoscale metal oxide comprises CuO and Cr₂O₃ nanocomposite powders having a Cu/Cr molar ratio between about 0.01 and about 0.8.

A fourth embodiment can include the method of the third embodiment, wherein the solution comprises hydrazine, and wherein the surfactant comprises cetyltrimethylammonium.

A fifth embodiment can include the method of the third or fourth embodiment, wherein the reducing agent comprises an aqueous solution of NaOH, urea, or a combination thereof, and wherein the surfactant comprises CTAB, oleyl amine, or any combination thereof.

A sixth embodiment can include the method of the second embodiment, wherein the one or more metal precursors comprise cobalt (II) nitrate and cerium (III) nitrate, wherein the nanoscale metal oxide comprises Co₃O₄—CeO₂ nanocomposite powders having a Co/Ce molar ratio between about 1:1 and about 16:1.

A seventh embodiment can include the method of the sixth embodiment, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.

An eighth embodiment can include the method of the second embodiment, wherein the one or more metal precursors comprise cerium (III) nitrate and manganese (II) nitrate, and wherein the nanoscale metal oxide comprises Ce₂O₃—MnO_(x) nanocomposite powders having a Ce/Mn molar ration between about 1:7 and about 5:7.

A ninth embodiment can include the method of the eighth embodiment, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.

A tenth embodiment can include the method of the second embodiment, wherein the one or more metal precursors comprise cerium nitrate, aluminum nitrate, and hexachloroplatinic acid.

An eleventh embodiment can include the method of the tenth embodiment, wherein the nanoscale metal oxide comprises Pt, Al₂O₃, and CeO₂, wherein the Pt is present in an amount between about 0.5-1.5 wt %, and wherein the CeO₂ is present in an amount of between about 10-40 wt %.

A twelfth embodiment can include the method of the tenth or eleventh embodiment, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.

A thirteenth embodiment can include the method of any of the first to twelfth embodiments, wherein forming the sensing layer comprises forming thin films or thick films using the nanoscale metal oxide.

In a fourteenth embodiment, a VOC sensor comprises a substrate, a plurality of leads disposed on the substrate, and a metal oxide nanocomposite film disposed in electrical contact with at least two of the plurality of leads.

A fifteenth embodiment can include the VOC sensor of the fourteenth embodiment, wherein the metal oxide nanocomposite film comprises a semiconductor film.

A sixteenth embodiment can include the VOC sensor of the fourteenth embodiment, wherein the metal oxide nanocomposite film comprises a nanostructured CuO—Cr₂O₃ composite.

A seventeenth embodiment can include the VOC sensor of the fourteenth embodiment, wherein the metal oxide nanocomposite film comprises a nanostructured Co₃O₄—CeO₂ composite.

An eighteenth embodiment can include the VOC sensor of the fourteenth embodiment, wherein the metal oxide nanocomposite film comprises a nanostructured CeO₂—MnO_(x) composite.

A nineteenth embodiment can include the VOC sensor of the fourteenth embodiment, wherein the metal oxide nanocomposite film comprises a nanostructured Pt—Al₂O₃—CeO₂.

A twentieth embodiment can include the VOC sensor of any of the fourteenth to nineteenth embodiments, wherein the nanocomposite film comprises a plurality of nanoscale metal oxide particles.

While various embodiments in accordance with the principles disclosed herein have been shown and described above, modifications thereof may be made by one skilled in the art without departing from the spirit and the teachings of the disclosure. The embodiments described herein are representative only and are not intended to be limiting. Many variations, combinations, and modifications are possible and are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention(s). Furthermore, any advantages and features described above may relate to specific embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages or having any or all of the above features.

Additionally, the section headings used herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or to otherwise provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings might refer to a “Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a limiting characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.

Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Use of the term “optionally,” “may,” “might,” “possibly,” and the like with respect to any element of an embodiment means that the element is not required, or alternatively, the element is required, both alternatives being within the scope of the embodiment(s). Also, references to examples are merely provided for illustrative purposes, and are not intended to be exclusive.

While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted or not implemented.

Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein. 

1-15. (canceled)
 16. A method for fabrication of VOC sensor, the method comprising: dissolving one or more metal precursors in a reagent to form a solution, adding a reducing agent to precipitate a metal oxide compound, subjecting the solution to acoustic energy, recovering a nanoscale metal oxide, and forming a sensing layer in a chemo-resistance sensor using the nanoscale metal oxide.
 17. The method of claim 16, further comprising: adding a surfactant to the solution prior to subjecting the solution to the acoustic energy.
 18. The method of claim 17, wherein the one or more metal precursors comprise copper nitrate and chromium nitrate, wherein the nanoscale metal oxide comprises CuO and Cr₂O₃ nanocomposite powders having a Cu/Cr molar ratio between about 0.01 and about 0.8.
 19. The method of claim 18, wherein the solution comprises hydrazine, and wherein the surfactant comprises cetyltrimethylammonium.
 20. The method of claim 18, wherein the reducing agent comprises an aqueous solution of NaOH, urea, or a combination thereof, and wherein the surfactant comprises CTAB, oleyl amine, or any combination thereof.
 21. The method of claim 17, wherein the one or more metal precursors comprise cobalt (II) nitrate and cerium (III) nitrate, wherein the nanoscale metal oxide comprises Co₃O₄—CeO₂ nanocomposite powders having a Co/Ce molar ratio between about 1:1 and about 16:1.
 22. The method of claim 21, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.
 23. The method of claim 17, wherein the one or more metal precursors comprise cerium (III) nitrate and manganese (II) nitrate, and wherein the nanoscale metal oxide comprises Ce₂O₃—MnO_(x) nanocomposite powders having a Ce/Mn molar ration between about 1:7 and about 5:7.
 24. The method of claim 22, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.
 25. The method of claim 17, wherein the one or more metal precursors comprise cerium nitrate, aluminum nitrate, and hexachloroplatinic acid.
 26. The method of claim 25, wherein the nanoscale metal oxide comprises Pt, Al₂O₃, and CeO₂, wherein the Pt is present in an amount between about 0.5-1.5 wt %, and wherein the CeO₂ is present in an amount of between about 10-40 wt %.
 27. The method of claim 25, wherein the reducing agent comprises NaOH, urea, or any combination thereof, wherein the surfactant comprises CTAB, olyel amine, or any combination thereof.
 28. The method of claim 16, wherein forming the sensing layer comprises forming thin films or thick films using the nanoscale metal oxide.
 29. A VOC sensor comprising: a substrate, a plurality of leads disposed on the substrate, and a metal oxide nanocomposite film disposed in electrical contact with at least two of the plurality of leads.
 30. The VOC sensor of claim 29, wherein the metal oxide nanocomposite film comprises a semiconductor film.
 31. The VOC sensor of claim 29, wherein the metal oxide nanocomposite film comprises a nanostructured CuO—Cr₂O₃ composite.
 32. The VOC sensor of claim 30, wherein the metal oxide nanocomposite film comprises a nanostructured Co₃O₄—CeO₂ composite.
 33. The VOC sensor of claim 29, wherein the metal oxide nanocomposite film comprises a nanostructured CeO₂—MnO_(x) composite.
 34. The VOC sensor of claim 29, wherein the metal oxide nanocomposite film comprises a nanostructured Pt—Al₂O₃—CeO₂.
 35. The VOC sensor of claim 29, wherein the nanocomposite film comprises a plurality of nanoscale metal oxide particles. 